1. Technical Field of the Invention
The present invention relates to an ink jet recording head for emitting an ink droplet from a nozzle aperture. More particularly, the present invention is directed to an ink jet recording head which emits an ink droplet by flexural oscillation according to an actuator, so that a part of a pressure generating chamber, which communicates with a nozzle aperture, expands and contracts.
2. Description of the Prior Art
Conventionally, there are two types of ink jet recording heads available: a piezoelectric vibrator type which mechanically deforms a pressure generating chamber to pressurize the ink and a bubble jet type which provides an exothermic element in a pressure generating chamber to pressurize the ink by the pressure of bubbles generated by the head of the exothermic element. The piezoelectric vibrator type recording head is further classified into two types: a first type of recording head using a piezoelectric vibrator displaced in the axial direction and a second type using a piezoelectric vibrator flexurally displaced. The first recording head can be driven at high speed and can record at high density, however, the manufacturing processes is complex because cutting work is required for machining a piezoelectric vibrator or three-dimensional assembly operations are required to fix the piezoelectric vibrator to a pressure generating chamber.
In contrast, the second type of recording head, provided with a pressure generating chamber and an actuator unit for pressurizing ink by a piezoelectric vibrator, can be formed by a ceramic baking technique, thus the manufacturing process is simpler than the manufacturing method of the first recording head. However, the whole recording head can become warped due to thermal stress. specifically, since a nozzle plate, for emitting ink pressurized by the actuator unit as an ink droplet, is formed by a metallic plate and the pressure generating chamber and actuator unit are formed by ceramic so that the recording head is constituted by integrating both via an adhesive layer, because of a difference in the coefficient of thermal expansion between ceramic and metal the head can warp. Warping may result in failure in emitting an ink droplet, and thus, deterioration of the printing quality.
To solve such problems, a thermal expansion characteristic adjusting member, having a section in the shape of reversed "C" and which does not prevent a piezoelectric vibrator from being vibrated, is fixed to the piezoelectric vibrator fixed to a surface of an actuator unit so as to prevent the whole recording head from being distorted because of a difference in thermal expansion as disclosed in Japanese published unexamined patent application No. H6-122197.
However, in such a recording head which uses a piezoelectric vibrator according to flexural oscillation, the area of the opening of a pressure generating chamber is larger to provide an adequate area for flexure, compared with a recording head using a piezoelectric vibrator in a longitudinal vibration mode. This has a problem that the recording density is deteriorated, compared with the first recording head.
To solve such a problem, a method has been adopted of using a silicon monocrystalline substrate for the base material, forming a passage such as a pressure generating chamber and a reservoir by anisotropic etching and forming a piezoelectric vibrator by film forming technique such as sputtering piezoelectric material. According to this method, an extremely thin elastic film can be formed and a pressure generating chamber and a piezoelectric vibrator can be formed precisely, so that the area of the opening of a pressure generating chamber can be minimized and recording density can be enhanced.
However, since the nozzle plate is still a metallic plate in order to maintain the machining precision of a nozzle aperture, there is a problem that the whole recording head is distorted because of the difference in thermal expansion as described above with respect to the second recording head in which a piezoelectric vibrator is fixed by baking. Such a problem can be solved by using a thermal expansion characteristic adjusting member disclosed in Japanese published unexamined patent application No. H6-122187; however, if a piezoelectric vibrator is constituted by sputtering piezoelectric material, a quantitatively higher electric field is applied in which the piezoelectric vibrator is thinner, compared with a piezoelectric vibrator constituted by baking a green sheet. If the above piezoelectric vibrators are driven at the same voltage, leakage current between driving electrodes increases and humidity in the atmosphere is absorbed and finally, dielectric breakdown is caused.